Television signals are typically transmitted to the viewing public using one of two different mediums, conventional airwaves ("broadcast television") or shielded transmission lines ("cable television"). There are differences in the signals between these two mediums, particularly in terms of the frequencies used. Consequently, modern television sets typically have some means of distinguishing between a cable television signal and a broadcast television signal. This means may take the form of an on-screen menu item or a manual switch operated by the user. It is desirable, however, to eliminate the need for a user to specify the type of signal being provided to a television set. In particular, it is desirable for a television set or other device receiving a television signal to have the ability to automatically identify the received signal as broadcast or cable.
In addition, certain cable television systems shift the frequency of the video and audio carriers from the standard frequencies for some or all television channels. This shifting may be done to reduce interference caused by the mixing of the carriers on the cable system. Two common schemes for shifting the carrier are the Incremental Coherent (related) Carrier (IRC) scheme and the Harmonically Related Carrier (HRC) scheme. Therefore, it is also desirable for a television set or other device receiving a television signal to have the ability to automatically distinguish between IRC, HRC, and standard cable television signals.